If you set obligatory parameters for any UI element or select options that doesn't mean that system will check for mandatory fields . The obligatory parameters will just display the red asterisk symbol for the UI element.Call the class for checking the mandatory fields using method

Use of Assistance Class in Webdynpro
Assistance class functionality in WDA Generally in webdynpro ABAP,we use to write all the coding part within the component itself but there is a provision with which we can write the necessary...